Insecticides kill their target insect species in a variety of ways. Two of the most commonly used classes of insecticide are the organophosphates (nerve gases) and the carbamates. These compounds act quickly (in a matter of hours), are lethal at low doses (parts per billion ), degrade rapidly (in hours to days) and leave few toxic residues in the environment . Organophosphates kill insects by inducing loss of control of the peripheral nervous system, leading to uncontrollable spasms followed by paralysis and, ultimately, death. This is often accomplished by a biochemical process called cholinesterase inhibition.
Most animals' nervous systems are composed of individual nerve cells called neurons. Between any two adjacent neurons there is always a gap, called the synaptic cleft; the neurons do not actually touch each other. When an animal senses something—for example, pain—the sensation is transmitted chemically from one neuron to another until the impulse reaches the brain or central nervous system. The first neuron (pre-synaptic neuron) releases a substance, known as a transmitter, into the synaptic cleft. One of the most common chemical transmitters is called acetylcholine. Acetylcholine then diffuses across the gap and binds with receptor sites on the second neuron (post-synaptic neuron). Reactions within the target neuron triggered by occupied receptors result in further transmission of the signal. As soon as the impulse has been transmitted, the acetylcholine in the gap is immediately destroyed by an enzyme called cholinesterase; the destruction of the acetylcholine is an absolutely essential part of the nervous process. If the acetylcholine is not destroyed, it continues to stimulate indefinitely the transmission of impulses from one neuron to the next, leading to loss of all control over the peripheral nervous system. When control is lost, the nervous system is first overstimulated and then paralyzed until the animal dies. Thus, organophosphate insecticides bind to the cholinesterase enzyme, preventing the cholinesterase from destroying the acetylcholine and inducing the death of the insect.
Some trade names for organophosphate insecticides are malathion and parathion. Carbamates include aminocarb and carbaryl. The carbamates produce the same effect of cholinesterase inhibition as the organophosphates, but the chemical reaction of the carbamates is more easily reversible. The potency or power of these compounds is usually measured in terms of the quantity of the pesticide (or inhibitor) that will produce a 50% loss of cholinesterase activity. Since acetylcholine transmission of nervous impulses is common to most vertebrates as well as insects, there is a great potential for harm to non-target species from the use of cholinesterase-inhibiting insecticides. Therefore the use of these insecticides is highly regulated and controlled. Access to treated areas and contact with the compounds is prohibited until the time period necessary for the breakdown of the compounds to non-toxic end products has elapsed.
[Usha Vedagiri ]
Connell, D. W., and G. J. Miller. Chemistry and Ecotoxicology of Pollution. New York: Wiley, 1984.